The present invention relates to a resist removing method, and specifically to a method of efficiently removing a resist which contains a high concentration of impurities and is applied before the injection of a high dose of ions onto a semiconductor wafer.
Generally, in the production of semiconductor devices, a high dose of ions is injected to some portions of a semiconductor wafer for the purpose of forming a highly conductive semiconductor area, etc. In this case, a resist is applied onto the surface of the semiconductor wafer to mask the area where the high dose of ions is not required, but the resist composition is often deteriorated badly. Therefore, conventional methods of removing the deteriorated resist composition in an aftertreatment cannot efficiently remove it only by a chemical reaction using 0.sub.2 plasma (oxygen gas plasma). Especially when the dose amounts to 1.times.10.sup.15 ions/cm.sup.2 or more, the resist composition cannot be removed efficiently, thereby resulting in defective removal.
Conventionally, to avoid the above described problems, the following methods are used to remove a resist composition. For example,
1. A method of accelerating the reaction of a resist with the O.sub.2 plasma by mixing H.sub.2 (hydrogen gas). PA1 2. A method of accelerating the reaction of a resist with the O.sub.2 plasma, etc. by applying a high frequency electric field to a semiconductor wafer. PA1 3. A method of forcing UV ozone obtained by the irradiation of the O.sub.2 plasma with far ultraviolet rays to react with a resist. PA1 1. The mixed gas of O.sub.2 and H.sub.2 may easily cause an explosion depending on its mixing ratio. PA1 2. The high frequency electric field applied to the semiconductor wafer 1 admits no physical or mechanical faults in the semiconductor wafer 1. PA1 3. In the method in which UV ozone reacts with the resist, the resist 2 cannot be removed as efficiently as shown in FIG. 1A to 1D. Therefore, in an aftertreatment, the residual resist must be removed by a wet etching process. This cannot avoid the reduction of insulation films such as thermal oxidation films applied on the surface of a semiconductor wafer. That is, the conventional resist removing methods still have problems in safe production (refer to 1 above) and reliable semiconductor devices (refer to 2 and 3 above).
FIGS. 1A through 1D show processes for explaining the conventional resist removing methods in which both the above described method 1 of mixing H.sub.2 and method 2 of applying a high frequency electric field are used in combination.
As shown in 1A, a resist 2 is selectively applied to the surface of a semiconductor wafer 1 made of such materials as Si (silicon) except areas where high dose of ions is injected.
Next, as shown in 1B, p-type impurities such as B (boron) and n-type impurities such as As (arsenic) or P (phosphorus) are injected in high density (represented by "+" in FIG. 1B) at a high temperature atmosphere above the semiconductor wafer 1 to which the resist 2 is selectively applied. High dose of ions is injected onto the exposed areas of the surface of the semiconductor wafer 1, that is, areas not covered with the resist 2. Simultaneously, the high dose of ions is injected onto the resist 2 itself, thereby causing the deterioration of the composition of the resist 2.
Next, as shown in FIG. 1C, the semiconductor wafer 1 to which the high dose of ions was injected is provided with the mixed gas of O.sub.2 and H.sub.2. Then, the resist 2 applied onto the semiconductor wafer 1 under the influence of a high frequency electric field (described later), chemically reacts with the mixed gas of O.sub.2 and H.sub.2, and is finally decomposed to CO.sub.2 (carbon dioxide) and H.sub.2 O (water).
As shown in FIG. 1D, the resist 2 selectively applied onto the surface of the semiconductor wafer 1 can be removed using a microwave ashing system 3. That is, the semiconductor wafer 1 is mounted on a sample table 3b provided inside a chamber 3a of the microwave ashing system 3, and supplied with a high frequency electric field by a high frequency power source 3c (oscillation frequency of 13.56 MHz). In the chamber 3a , the mixed gas of O.sub.2 and H.sub.2 introduced to the chamber 3a is excited by the microwave (oscillation frequency of 2.45 GHz) introduced from a directing chamber 3d. As a result, O.sub.2 and H.sub.2 are changed to O.sub.2 plasma (including oxygen radicals, etc.) and H.sub.2 plasma respectively. The resultant plasma is attracted to the semiconductor wafer 1 by the influence of the high frequency electric field generated by the high frequency power source 3c, and actively reacts with the component of the resist 2.
Thus, in the above described process, the composition of the resist 2 is forced to chemically react with the actions of H.sub.2 and the high frequency electric field although the composition of the resist 2 selectively applied onto the surface of the semiconductor 1 is badly deteriorated by the injection of the high dose of ions. As a result, the resist 2 can be efficiently removed from the semiconductor wafer 1, thereby preventing defective removal.
Although the resist removing method shown in FIG. 1A to 1D enables the resist 2 to be efficiently removed from the semiconductor wafer 1, there are the following problems with the method.